The present invention relates generally to a coated paper product having high gloss and brightness and the method of manufacturing such a product. In particular, the invention relates to a process for manufacturing a coated paper product with a surface comparable to a cast coated surface, that may be used, for example, as the facing sheet of a pressure sensitive laminate. In addition to this intended use, the product of the present invention is suitable for a variety of other printing and converting operations such as metallizing, foil laminating and printing, security label applications and, specialty packaging as well as upscale gift wrap and labels.
Such paper products have in the past been produced almost exclusively by a cast coating process. During cast coating, gloss development relies on a replication of the mirror-like finish on a dryer roll, as the applied coating is dried. However, production rates for the cast coating process are considerably slower than the production of coated paper on a high speed papermachine. Thus it would be desirable and advantageous to develop a high speed coating process that could be used to produce a cast coated surface on paper Examples of the cast coating process are disclosed in prior U.S. Pat. Nos. 4,241,143 and 4,301,210.
Another method for producing high gloss paper is disclosed in U.S. Pat. No. 5,360,657. In this patent, a process is disclosed in which a thermoplastic polymeric latex having a second order transition temperature of at least 80 degrees C., and an average particle size smaller than 100 microns is applied to paper which is subsequently calendered to produce high gloss. Other methods for producing high gloss paper include the application of a glossy overprint varnish onto a previously coated substrate. However, in the latter case, the glossy surface produced is not generally useful for offset printing because of the excessive ink drying time required.
It is also known, as disclosed for example in PCT published application WO 98/20201, that a printing paper having high brightness and gloss can be manufactured by applying to paper a coating comprising at least 80 parts precipitated calcium carbonate and at least 5 parts of an acrylic styrene copolymer hollow sphere plastic pigment. The published application also notes that a finishing step using a calender is required to achieve the gloss development, but the method of calendering is deemed to be not restrictive. Likewise, in an article entitled xe2x80x9cLightweight Coated Magazin Papers,xe2x80x9d published in the Jul. 5, 1976 issue of the magazine PAPER, Vol. 186, No. 1, at pages 35-38, a relationship between calendering and the use of plastic pigments in coatings is disclosed. The article notes that polymers such as polystyrene are thermoplastic and pressure sensitive, and a pigment based on polystyrene will exhibit a high degree of calendering response.
These and other publications including an article entitled xe2x80x9cLight Reflectance of Spherical Pigments in Paper Coatings,xe2x80x9d by J. Borch and P. Lepoutre, published in TAPPI, February 1978, Vol. 61, No. 2, at pages 45-48; an article entitled xe2x80x9cPlastic Pigments in Paper Coatings,xe2x80x9d by B. Aluice and P. Lepoutre, published in TAPPI, May 1980, Vol. 63, No. 5, at pages 49-53; and an article entitled xe2x80x9cHollow-Sphere Polymer Pigment in Paper Coating,xe2x80x9d by J. E. Young, published in TAPPI, May 1985, Vol. 68, No. 5, at pages 102-105, all recognize the use of polymer pigments in paper coatings, but none of these publications disclose the unique combination of coating formulation and finishing conditions disclosed herein.
The present invention relates generally to a coated paper product and method of producing it. More particularly, the invention relates to a coated paper product that can be manufactured on a high speed papermachine and still achieve a high gloss, high brightness surface typical of cast coated paper.
The coatings disclosed herein for practicing the present invention include conventional inorganic pigments such as clay and calcium carbonate in conjunction with elevated amounts of thermoplastic polymer latex beads. The beads are either hollow or solid in composition. Upon applying these coatings onto an uncoated but smoothened basestock, or onto a precoated basestock, it is possible to achieve a high gloss and smoothness with good printing properties when the coated surface is finished in a calendar device such as a supercalender containing heated rolls.
Paper produced with the high plastic pigment content coating preferred for the present invention is suitable for printing using conventional printing methods including sheet-fed litho offset, flexography, rotogravure and web offset.
The high gloss coatings of the present invention comprise standard coating pigments such as clay, ground or precipitated calcium carbonate, titanium dioxide and elevated amounts of plastic pigment. While the content of plastic pigment in the coating formulation plays a significant role in achieving high gloss, an equally important factor which contributes to the desired finished paper properties is the surface area of the paper which comprises plastic pigment. SEM micrographs of coated paper surfaces were analyzed for plastic pigment spheres on the surface of the paper. The number of spheres were counted and an approximate percent of the total area of the sheet was calculated. The results showed an effect of coating speed/coating solids on plastic sphere areas as a percent of surface area. It was noted that as coating speed increased, a greater amount of surface area was filled with plastic spheres producing greater gloss development. The reason for this is not clear, but one possible explanation is that at increasingly higher coating speeds, drying is more intense, and as water is driven from the coated surface during drying, the plastic spheres (being of equivalent density when filled with water and of lower density as water is evaporated), are transported through the coating to the surface of the coated paper. Therefore to achieve a target gloss, lower amounts of plastic pigment may be used when the method and speed of the coating application is taken into account.
In addition, the size of the plastic pigment plays a role in the performance of the coating, vis-a-vis gloss development. For example, paper gloss achieved with a 0.45 micron diameter solid sphere plastic pigment is not as good as that obtained with a hollow sphere plastic pigment when the percent of surface area is taken into consideration. It is postulated that this ineffectiveness may be related to the diameter and curvature of the sphere presented to incoming light and subsequent light scattering. For example, five 0.45 micron diameter solid spheres will occupy approximately the same space as a 1.0 micron diameter hollow sphere. However, hollow spheres can flatten upon calendering and create a plurality of multiple flat surfaces for more efficient light reflection and gloss development. Meanwhile the use of a 0.20 micron diameter solid sphere plastic pigment will more closely simulate a flatter surface than the 0.45 micron diameter spheres because approximately twenty five 0.20 micron diameter spheres will occupy the same space as a single 1.0 micron diameter hollow sphere.
In summary, the preferred coating formulation for achieving the results of the present invention comprises from 46-60% calcium carbonate, 0-33% coating clay, 0-5.5% titanium dioxide and from 14-35% plastic pigment. The preferred plastic pigment is a hollow sphere plastic pigment having a particle size of up to 1.0 micron diameter selected from the group consisting of polystyrene, acrylics and methaecrylates. However, solid sphere plastic pigments ranging from 0.20-0.45 micron diameter may be substituted for the hollow sphere pigment or blended with the hollow sphere pigment as desired.
The preferred finishing step in the manufacture of the high gloss coated paper disclosed herein involves a supercalender apparatus operated at speeds ranging from about 800-2800 fpm, and at calender loads of from about 1500-2000 phi, with one or more rolls heated to a temperature of from about 100-240 degrees F. It should be noted, however, that gloss development equivalent to that obtained with a super-calender apparatus may be obtained with a gloss calender or soft roll calender under appropriate operating conditions.